Signal offset cancellation

ABSTRACT

Techniques and circuitry are provided for programmably controlling signal offsets in integrated circuitry. In one embodiment, an integrated circuit includes a signal offset cancellation circuit that is programmably selected to control the offset of signals on either one input/output or another input/output of an amplifier circuit. In one embodiment, a logic circuit is used to selectively couple a bank of current sources to one input/output or another input/output of a differential amplifier through a switching circuit. The bank of current sources may employed to control the signal offset on either input/output, or may be decoupled from all of the inputs/outputs when signal offset cancellation is not required.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.11/245,581, filed Oct. 6, 2005, and entitled “Programmable Logic EnabledDynamic Offset Cancellation” and U.S. patent application Ser. No.11/323,571, filed Dec. 29, 2005, and entitled “Comparator OffsetCancellation Assisted by PLD Resources,” both of which are incorporatedby reference herein for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to techniques for controlling signaloffsets, and more particularly, to techniques for correcting signaloffsets associated with integrated circuit buffers and amplifiers usingprogrammable resources.

Generally, interface circuitry such as input and output buffer circuitsare used to amplify and/or condition signals for detection ortransmission. In the case of an input buffer in a telecommunicationsystem, the input buffer circuit receives an input signal that hastypically undergone degradation and attenuation as it propagated througha transmission link. The function of the input buffer is therefore toamplify and recondition the received signal, and in some cases toprovide frequency equalization, so that the receiver circuitry canproperly resolve the incoming bits. In the case of an output buffer, thecircuit is typically required to drive an output signal at theappropriate levels for a given transmission link. In either case, anysignal offset that may be caused by the buffer circuitry can contributeto operational error. Signal offsets, typically voltage offsets, reducethe available timing margins needed to resolve incoming data bits. Thiscauses an increase in the bit error rate (BER) of the receiver circuit.In the case of output buffers, offsets cause undesirable duty cycledistortion for the output signal.

Various offset cancellation techniques have been developed to eliminateor reduce the adverse effects of signal offsets. For example, indifferential circuits, input and output buffer circuits often includeselectable current sources coupled to outputs of a differentialamplifier circuit. Each current source is typically connected inparallel with a respective transistor output of the transistor pair.Such an arrangement allows the current source and respective transistorto form a voltage divider. The voltage divider is used to adjust avoltage offset with respect to the amount of fixed current being drawnby the current source. Unfortunately, such a conventional arrangementoften adds additional unnecessary load to the transistor output whenvoltage offset is not required. For example, controlling the voltageoffset may be unnecessary where the voltage offset may be designedwithin a tolerance range or part of a given circuit that is unaffectedby voltage offsets.

Moreover, as such selectable current sources are generally run parallelto one another, and draw different amounts of fixed current, the amountof change in voltage offset is adjustable by selecting different currentsources alone or in parallel to achieve the desired offset voltageamount. Unfortunately, even with precise process control during circuitmanufacturing, process variations often introduce differences in thedifferential circuit components. Such differences often translate intovariations in offset signal control with respect to each differentialinput/output. In addition, the circuitry used to control the currentsources and the current sources themselves consumes valuable circuitspace on the die.

There is therefore a need for circuits and methods to reduce oreliminate signal offsets when desired in order to improve integratedcircuit operational performance while requiring less die space andcomplexity than conventional signal offset correction circuits.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention pertain to techniques and circuitryto control signal offsets in integrated circuits and systems. Generally,the present invention employs programmable resources to correct foroffsets without increasing circuit complexity and loading conditions.The programmability of the offset cancellation technique according tothe present invention allows for adjusting the signal offset such thateither inputs or outputs of a differential amplifier may employ signaloffset correction.

In one embodiment, the present invention provides an integrated circuithaving a buffer with an offset cancellation circuit. The offsetcancellation circuit includes a bank of parallel current sources thatare selectably connected to one output or another output of thedifferential amplifier portion of the buffer through a switchingcircuit. The bank of parallel current sources may be programmablycontrolled to change the signal offset for one output of thedifferential amplifier or the other, or to be decoupled from eitheroutput.

A better understanding of the nature and advantages of the presentinvention can be gained from the following detailed description and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of one exemplary embodiment of anoffset cancellation circuit according to the present invention;

FIG. 2 is a simplified circuit diagram of a buffer with offsetcancellation circuitry according to an exemplary embodiment of thepresent invention;

FIG. 3 is a simplified circuit diagram of a carry chain circuitaccording to an exemplary embodiment of the present invention;

FIG. 4 is a simplified circuit diagram of a carry chain circuit and thecarry chain path according to an exemplary embodiment of the presentinvention;

FIG. 5 is a simplified block diagram of a programmable logic device thatcan embody the techniques of the present invention;

FIG. 6 is simplified a block diagram of an electronic system that canimplement embodiments of the present invention; and

FIG. 7 is a flow diagram for a method of correcting signal offsetsaccording to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention pertains to correcting signal offsets usingprogrammable resources. Generally, signal offsets may be categorized asDC voltage offsets or as average voltage offsets attributable to ACwaveforms. For example, when integrated circuits are DC coupled, signaloffsets may be described in terms of a DC voltage offset. When theintegrated circuits are AC coupled, the signal offsets may becategorized as a voltage offset due to an averaging of the AC waveformtransmitted therebetween. For example, in the case of an AC coupleddifferential amplifier, the AC offset averages converge to an averagecommon-mode offset voltage.

Signal offsets are caused by variations and mismatches in transistorsand other integrated circuit components. For example, a buffer circuitmay include a differential amplifier with a differential input pair oftransistors. Mismatches in physical and electrical characteristics ofthe transistors forming the differential input pair can cause signaloffsets. The present invention provides various techniques forcorrecting these types of offsets. While the invention is describedherein in the context of various differential input buffers, thoseskilled in the art will appreciate that the techniques described hereincan be applied to single-ended circuits as well as output buffers andany other circuitry that can benefit from offset cancellation.

Referring to FIG. 1, there is shown a high level block diagram of anintegrated circuit 100 with programmable offset cancellation accordingto one exemplary embodiment of the present invention. Integrated circuit100 includes an input buffer 110, switch 120, and programmable resourcessuch as signal select logic 130 and offset signal control logic 150. Inone embodiment, input buffer 110 receives a differential input signal atinput 102 and input 104. Input buffer 110 amplifies the input signal andcouples it to, for example, another buffer amplifier, driver circuit,receiver block, and the like, via outputs 106 and 108, examples of whichare described below. Input buffer 110 includes offset signal_input 111and an offset signal_B input 112. During operation of integrated circuit100, either offset signal_A input 111 or offset signal_B input 112 isused to control the offset of buffer 110.

In one configuration, a programmable offset signal source 140 generatesan offset signal 142 (e.g., a voltage and/or a current signal) inresponse to, for example, an offset cancellation algorithm. Offsetsignal 142 is coupled through switch 120 to 110 at either offsetsignal_A input 111 or offset signal_B input 112. In one configuration,in response to a select signal 132, switch 120 decouples offset signal142 from buffer 110. Switch 120 may be any type of switch capable ofcoupling and decoupling offset signal 142 from buffer 110.

In one embodiment, integrated circuit 100 includes signal select logic130. Based on a desired logic setting from, for example, a ProgrammableLogic Device (PLD), signal select logic 130 generates select signal 132which is coupled to switch 120 for control thereof. Signal select logic130 can be configured using a variety of methods to select whichdifferential node in buffer 110 receives offset signal 142 (e.g., offsetsignal_A input 111 or offset signal_B input 112). One method, forexample, utilizes a calibration process which first applies offsetsignal 142 to either offset signal_A input 111 or offset signal_B input112 to buffer 110. The offset at the output of buffer 110 is thenmeasured to determine which output 106 or 108 to adjust. The calibrationprocess also measures the amount of offset which indicates the magnitudeof offset signal 142 required to correct for the offset. Advantageously,signal select logic 130 allows the user to perform this calibrationprocess under system control, for example, whenever there is no datatraffic, as needed. A similar process can be performed if input data isAC coupled. For AC coupled inputs, when there is no toggling of thesignal, the DC level converges to the common mode level allowing offsetsto be measured by detecting the switching point for signal transition atan output of a receiver block, for example.

Offset signal 142 may include a current signal, a voltage signal, ordigital data signal configured to directly or indirectly adjust thelevel of offset of signals on outputs 106 and 108. For example, offsetsignal 142 may be a current signal generated by a current source versionof offset signal source 140, embodiments of which are described below.Such a current signal may be adjusted in magnitude to control the signaloffset present on either output 106 or output 108 during circuitoperation as described below with respect to FIG. 2 in order to, forexample, correct the signal offset. In another embodiment, offset signal142 may be used to indirectly control the amount of voltage and/orcurrent offset of buffer 110. For example, it is contemplated thatbuffer 110 may be configured such that offset signal_A input 111 andoffset signal_B input 112 are responsive to digital signals. In thisembodiment, offset signal 142 may be a digital word digitallycontrolling an offset of buffer 110.

In one embodiment, offset signal source 140 generates offset signal 142in response to offset control signal 152. Offset control signal 152 maybe generated from offset signal control logic 150, or may be suppliedexternally, from for example, and external signal input. Signal controllogic 150 is configured to programmably control the magnitude of theoffset signal 142 provided by offset signal source 140. In oneembodiment, offset control signal 152 may be used to directly orindirectly control the magnitude of offset signal 142 supplied to switch120. For example, offset control signal 152 may be a current controlsignal capable of adjusting the magnitude of a current signal or voltagesignal of offset signal 142. In one embodiment, to minimize possiblenoise bleed though and/or extraneous power consumption, signal offsetsource 140 may be disabled when offset signal 142 is not needed.

Advantageously, to provide system flexibility signal control logic 150may be preset to control the magnitude of offset signal 142. Forexample, for a given system that requires a voltage offset within apredetermined tolerance, signal control logic 150 may be preset toadjust offset signal 142 which adjusts the voltage offset of buffer 110to within the desired tolerance limits. Thus, the provision of signalselect logic 130 and signal control logic 150 simplifies the front-endcircuitry for offset cancellation and allows the user to develop anoffset cancellation algorithm that cancels offsets based on the needs ofa given application.

FIG. 2 is a simplified circuit diagram of one exemplary embodiment ofintegrated circuit 100 of FIG. 1. In this embodiment, buffer 110includes a differential transistor pair 210 made up of a pair of inputtransistors 204 and 206. The gate terminals of input transistors 204 and206 are coupled to inputs 104 and 106, respectively. Outputs 106 and 108are respectively coupled to the drain terminals of input transistors 204and 206. The source of each transistor 204 and 206 couples to acommon-source node. Optional transistors 214 and 216 may be coupled inseries to the common-source node and controlled via control signals 206and 208, respectively, to enable and disable, and control the overallcurrent flow through transistor pair 210.

In one embodiment, differential transistor pair 210 may be coupled to adriver circuit 220 via outputs 106 and 108. Driver circuit 220 may be acommon mode driver as illustrated, converting outputs 106 and 108 into acommon mode output signal 232. In one embodiment, when a voltage offsetis present in such a differential transistor pair 210, for zero inputdifferential voltage applied to inputs 104 and 106, there will be anon-zero output voltage either in the negative direction or the positivedirection, e.g., common mode output signal 232 will be non-zero.

In one embodiment, switch 120 may include switches 222 capable ofprogrammably coupling either output 106 or output 108 to offset signal142. For example, switches 222 may be transistor pass-gates, ortransmission gates, capable of coupling offset signal 142 to eitheroutput 106 or output 108. Switches 222 may be formed as part of a PLDcircuit. Switches 222 may be programmably controlled via enableterminals responsive to, for example, select logic signal 132.

If an offset correction is needed, switch 120 may be controlled viaselect signal 132 to couple either output 106 or output 108 throughswitches 222 to offset signal 142. If offset correction is not needed,switch 120 is controlled via select signal 132 to decouple output 106and output 108 from offset signal 142.

During operation, signal select logic 130 may generate select signal 132in response to internal or external signals. For example, in oneembodiment signal select logic 130 may generate select signal 132 inresponse to offset control signal 152. For example, select signal 132may be derived from an offset control signal 152 having, for example, aM-bit word A[N:0] 232, where N is a one or more, that controls offsetsignal source 140. In this embodiment, signal select logic 130 mayinclude combinatorial logic 236 that employs a most-significant-bit(MSB) 234 of M-bit word 232 to select which output 106 or 108, if any,is coupled to offset signal 142. This is advantageous, as switch controlsignal 232 may be derived from the same M-bit word 234 used to controloffset signal source 140, thereby saving logic resources.

In one embodiment, offset signal source 140 includes a bank of offsetcurrent sources 244 coupled in parallel and coupled to switch 120 viacurrent select switches 246. The bank of offset current sources 244would typically be implemented by n-channel (or p-channel) transistorswhose gates are connect to a bias voltage. The bank of offset currentsources 244 may be selected in any combination to program a plurality ofcurrent levels as needed. For example, the bank of offset currentsources 244 may be multiples of a desired current increment offset. Asillustrated, current sources 244 may be configured as 1×, 2×, 3×, . . .fX times the current increment I_(offset), where f represents aplurality of multipliers. Advantageously, offset current sources 244 maybe preset to control the current value of offset signal 142 for a numberof given applications.

In order to connect one or more individual current sources 244 to switch120, current select switches 246 may be formed from a series ofpass-gates controlled by internal and/or external signals. For example,signal control logic 150 may employ a digital word such as digital wordC[0:N] to select which current sources 244 are to be used for a givenoffset signal 142. In one example, signal control logic 150 may includecombinatorial logic 154 capable of generating control signals C[0:N] inresponse to M-bit word 232.

In one embodiment, when offset correction is required on either output106 or output 108, combinatorial logic 154 may be configured to minimizethe number offset current sources 244 needed to generate the desiredoffset cancellation signal. FIG. 3, illustrates one embodiment ofcombinatorial logic 154A capable of generating control signals OC[5:0]in response to 3-bit word 232A. In this illustration, 3-bit word 232A isprovided by a three-bit binary bus RLF_OS[2:0] coupled to NAND gates302. Table one illustrates the sequence of which NAND-gate zero thoughNAND-gate one is selected upon a binary input value. As illustrated intable one, each NAND gate, zero though five, when selected, will selecta corresponding output signal OC[0], OC[1], OC[2], OC[3], OC[4], OC[5].These output signals OC[5:0] may be used to enable the 1×, 2×, 3×, etc.offset cancellation current sources 244.

TABLE 1 RLF_OS[2] RLS_OS[1] RLF_OS[0] Selected 0 0 0 NANDgate 0 0 0 1NANDgate 1 0 1 0 NANDgate 2 0 1 1 NANDgate 3 1 0 0 NANDgate 4 1 0 1NANDgate 5

In one embodiment, combinatorial logic 154A forms a carry chain 302 toallow such a digital word to control more than one current source 244.For example, when RLF_OS[2:0] is digital word 000, NAND-gate 0 isselected and therefore OC[0] is selected. When RLF_OS[2:0] is digitalword 001, NAND-gate one is selected and NANDgate zero is not selected.However, through carry chain 302 NAND-gate one selects OC[0]. In thiscase current sources controlled by OC[0] and OC[1] are selected.Similarly, when RLF_OS[2:0] is digital word 010, through carry chain 302control signals OC[2], OC[1], OC[0] are selected thereby selectingcurrent sources controlled by OC[2], OC[1], OC[0]. In other words, carrychain 302 enables control signals OC[n], OC[n−1], OC[n−2], . . . ,OC[1], OC[0] where the binary input value equals n.

FIG. 4 illustrates a path 402 of carry chain 302 of FIG. 3, which allowsthe deployment of available current sources 244 to maximize the amountof offset cancellation, while reducing the number of current sources andlogic typically needed in conventional signal offset cancellationcircuitry. In one embodiment, carry chain 302 may be used to reduce thenumber of current sources 244 required for a given offset cancellationrequirement. For example, consider the case where for a current offsetvalue of 1×, 2×, 4×, or 8× an I_(offset) value, a conventional directbinary decoding circuit would require 1+2+4+8=15 number of currentsources. Using the carry chain process of the present invention thetotal number of current sources 244 would be 1+1+2+4=8. Table twoillustrates one embodiment of a process using such a carry chain 302.

TABLE 2 Binary Input Value Carry Chain Process n = 0 select 1X n = 1select new 1X and previous 1X to obtain 2X select new 2X n = 2 andprevious 1X + 1X to obtain 4X n = 3 select new 4X and previous 1X + 1X +2X to obtain 8XIn this illustration, for a 3-bit digital word, using carry chain 302 ofthe present invention, for a digital word value of zero, one currentsource 244 is selected. For a digital word value of one, the carry chainselects a 1× current source 244 and another 1× current source 244. For adigital word value of two, the carry chain process selects the 1×current source 244, the other 1× current source 244, and a 2× currentsource 244. For a digital word value of three, the carry chain processselects the 1× current source 244, the other 1× current source 244, the2× current source 244, and a 4× current source 244.

While the various programmable logic enabled offset cancellationtechniques described herein can be employed in any type of integratedcircuit or system, they are particularly well suited for programmablelogic devices (PLDs) or field programmable gate arrays (FPGAs). This isso because PLDs and FPGAs provide powerful programmability that can veryefficiently implement different aspects of the present invention by anyoptimized combination of hardware and software. For example, the entireintegrated circuit 100, or portion thereof, shown in FIG. 1, can beimplemented by a complex PLD wherein the integrated circuit 100 mayinclude any one of a number of typical transceiver circuits such asclock data recovery (CDR), switches, dynamic phase adjustment (DPA),serializer-deserializer, phase locked loop or delay locked loopcircuitry and the like including clock networks. These circuit blocksmay be implemented by hardwired circuitry while signal select logic 130and signal control logic 150 is contained in the programmable core ofthe PLD. Such an implementation allows the user to create an offsetcancellation algorithm that may be invoked by the system upon power-up,reset or initialization, during system idle time or when low bit errorrate is detected in a given channel.

The PLD implementation allows the system or the user to customize theoffset cancellation scheme for the needs of the particular application.For example, in telecommunication applications, for channels that run ata lower data rate, the invention allows saving area and power by notenabling offset cancellation altogether. The present invention alsoprovides for technology migrations from one generation to the nexteasier since it eliminates the need to design a custom signal offsetcircuit for each PLD design. The invention can be further extended tothe entire link wherein not only the offset of the receiver can becancelled, but offsets associated with the physical layer andtransmitter can be cancelled when both ends of the link are under thecontrol of the PLD user.

FIG. 5 is a simplified partial block diagram of one example of PLD 500that can implement aspects of the present invention. It should beunderstood that the present invention can be applied to numerous typesof integrated circuits including programmable logic integrated circuits,field programmable gate arrays, mask FPGAs, and application specificintegrated circuits (ASICs) or application specific standard products(ASSPs) that provide programmable resources. Referring to FIG. 5, PLD500 includes a two-dimensional array of programmable logic array blocks(or LABs) 502 that are interconnected by a network of column and rowinterconnects of varying length and speed. LABs 502 include multiple(e.g., 10) logic elements (or LEs).

An LE is a programmable logic block that provides for efficientimplementation of user defined logic functions. A PLD has numerous logicelements that can be configured to implement various combinatorial andsequential functions. The logic elements have access to a programmableinterconnect structure. The programmable interconnect structure can beprogrammed to interconnect the logic elements in almost any desiredconfiguration.

PLD 500 also includes a distributed memory structure including RAMblocks of varying sizes provided throughout the array. The RAM blocksinclude, for example, 512 bit blocks 504, 4K blocks 506, and a block 508providing 512K bits of RAM. These memory blocks can also include shiftregisters and FIFO buffers.

PLD 500 further includes digital signal processing (DSP) blocks 510 thatcan implement, for example, multipliers with add or subtract features.I/O elements (IOEs) 512 located, in this example, around the peripheryof the device support numerous single-ended and differential I/Ostandards. These I/O elements 512 may include differential input oroutput buffers with offset cancellation circuitry of the type shown inFIGS. 1-5. PLD 500 can additionally provide transceiver functionalityfor telecommunication applications. In the exemplary embodiment shown inFIG. 5, PLD 500 includes one or more transceiver blocks 520. Transceiverblocks 520 may include integrated circuit 100 of FIG. 1 and mayimplement offset cancellation techniques as described herein. It is tobe understood that PLD 500 is described herein for illustrative purposesonly and that the present invention can be implemented in many differenttypes of PLDs, FPGAs, and the like.

While PLDs of the type shown in FIG. 5 provide many of the resourcesrequired to implement system level solutions, the present invention canalso benefit systems wherein a PLD is one of several components. FIG. 6shows a block diagram of an exemplary digital system 600, within whichthe present invention can be embodied. System 600 can be a programmeddigital computer system, digital signal processing system, specializeddigital switching network, or other processing system. Moreover, suchsystems can be designed for a wide variety of applications such astelecommunications systems, automotive systems, control systems,consumer electronics, personal computers, Internet communications andnetworking, and others. Further, system 600 can be provided on a singleboard, on multiple boards, or within multiple enclosures.

System 600 includes a processing unit 602, a memory unit 604 and an I/Ounit 606 interconnected together by one or more buses. According to thisexemplary embodiment, a PLD 500 is embedded in processing unit 602. PLD500 can serve many different purposes within the system in FIG. 6. PLD500 can, for example, be a logical building block of processing unit602, supporting its internal and external operations. PLD 500 isprogrammed to implement the logical functions necessary to carry on itsparticular role in system operation. PLD 500 can be specially coupled tomemory 604 through connection 610 and to I/O unit 606 through connection612.

Processing unit 602 can direct data to an appropriate system componentfor processing or storage, execute a program stored in memory 604 orreceive and transmit data via I/O unit 606, or other similar function.Processing unit 602 can be a central processing unit (CPU),microprocessor, floating point coprocessor, graphics coprocessor,hardware controller, microcontroller, programmable logic deviceprogrammed for use as a controller, network controller, and the like.Furthermore, in many embodiments, there is often no need for a CPU.

For example, instead of a CPU, one or more PLDs 500 can control thelogical operations of the system. In an embodiment, PLD 500 acts as areconfigurable processor, which can be reprogrammed as needed to handlea particular computing task. Alternately, programmable logic device 500can itself include an embedded microprocessor. Memory unit 604 can be arandom access memory (RAM), read only memory (ROM), fixed or flexibledisk media, PC Card flash disk memory, tape, or any other storage means,or any combination of these storage means.

FIG. 7 is a flow diagram of a method 700 of canceling signal offsetsaccording to one embodiment of the present invention. Method 700 startsat step 702, when for example, integrated circuit 100 is employed tocancel signal offsets in a given transmission link. At step 704, adetermination is made whether or not offset cancellation is required. Ifthere is no requirement for offset cancellation, then at step 706, theoffset signal is decoupled from all of the signals inputs/outputs. Forexample, switch 120 decouples offset signal source 140 and offset signal142 from buffer 110. This is advantageous as conventional offsetcancellation circuitry typically adds an impedance load to the bufferinputs/outputs. However, at step 704, if a offset cancellation isdesired, then method 700 proceeds to step 708.

At step 708, method 700 determines which signal to apply offsetcancellation. For example, switch 120 may be set to apply offsetcancellation to either output 106 or 108. In one embodiment, the choiceis programmably made by a PLD which selects to which output 106 or 108to apply offset cancellation signal 142. In one embodiment, this choicemay be predetermined during a design phase, manufacturing phase, testingphase, etc., for a given buffer circuit. For example, during a testingphase, a given lot of dies may require that an offset cancellationsignal be applied to output 106 to cancel signal offsets associated withthat particular lot of dies.

At step 710, the magnitude of the offset signal is established. In oneembodiment, one or more of a plurality offset values are employed toadjust the magnitude of the offset signal to meet the needs of a givensystem. For example, consider a case where to meet a signal offsettolerance for a desired offset voltage of an output signal, a currentamount is required that is two times a current value of I_(offset).Method 700 sets the current amount to two times the current value ofI_(offset) to generate the desired voltage offset of the output signal.

At step 712, the offset signal is coupled to either the designatedoutput signal. For example, offset signal 142 may be coupled to eitheroutput 106 or output 108. In one embodiment, the same signals used todetermine the magnitude of the offset cancellation may be used to choosewhich output to offset. This is advantageous, as it allows forsimplified programming. Method 700 ends at step 714.

The present invention thus provides various techniques for offsetcancellation that is enabled by programmable logic. While the aboveprovides a detailed description of various embodiments of the invention,many alternatives, modifications, and equivalents are possible.Therefore the scope of this invention should not be limited by thespecific embodiments described above, and should instead be determinedwith reference to the appended claims along with their full scope ofequivalents.

1. An integrated circuit comprising: a signal offset cancellationcircuit; a switching circuit having an output coupled to an input of thesignal offset cancellation circuit; a first transistor having a firstcurrent-carrying terminal, a second current-carrying terminal, and afirst signal terminal, wherein the first current-carrying terminal iscoupled to an input of the switching circuit; a second transistor havinga third current-carrying terminal, a fourth current carrying terminal,and a second signal terminal, wherein the third current-carryingterminal is coupled to another input of the switching circuit, whereinan output of the signal offset cancellation circuit is selectablycoupled to either the first current-carrying terminal or the thirdcurrent-carrying terminal through the switching circuit; signal selectlogic that controls the switching circuit and receives N control bits asinput; and offset signal control logic that controls the signal offsetcancellation circuit and receives a subset of the N control bits asinput.
 2. The integrated circuit of claim 1, wherein the signal offsetcancellation circuit comprises a plurality of current sources coupled inparallel.
 3. The integrated circuit of claim 1, wherein the signaloffset cancellation circuit comprises a programmable logic circuit. 4.The integrated circuit of claim 1, wherein the integrated circuitcomprises a differential amplifier comprising the first transistor andthe second transistor.
 5. The integrated circuit of claim 1, wherein thefirst transistor comprises an NMOS transistor or a PMOS transistor. 6.The integrated circuit of claim 1, wherein the switching circuit iscapable of decoupling the signal offset cancellation circuit from boththe first current-carrying terminal and the third current-carryingterminal.
 7. The integrated circuit of claim 1, wherein the signaloffset cancellation circuit generates a offset signal capable ofcontrolling the signal offset of the first current-carrying terminal orthe first current-carrying terminal.
 8. The integrated circuit of claim1, wherein the subset of the N control bits consists of N−1 controlbits, and wherein the remaining bit not in the subset is used to controlwhich current-carrying terminal is coupled with the output of the signaloffset cancellation circuit.
 9. The integrated circuit of claim 8,wherein the remaining bit is the most significant bit of the N controlbits.
 10. The integrated circuit of claim 8, wherein for one value ofthe remaining bit and for one value of the N−1 control bits, the signalselect logic decouples both of the first and third current-carryingterminals from the output of the signal offset cancellation circuit. 11.A programmable logic circuit, comprising: a transistor pair having afirst signal input, a second signal input, and a first signal output,and a second signal output; and a signal offset cancellation circuitselectively coupled to either the first signal output or the secondsignal output, wherein the signal offset cancellation circuit is capableof selectively providing an offset signal to cancel signal offsetsoutputted from either the first signal output or the second signaloutput; and signal control logic configured to control the magnitude ofthe offset signal, wherein the signal offset cancellation circuitcomprises a plurality of offset current sources capable of generatingthe offset signal, and wherein the signal control logic controls thesignal offset cancellation circuit to use a first set of offset currentsources to obtain a first magnitude of the offset signal and controlsthe signal offset cancellation circuit to use the first set of offsetcurrent sources plus one or more additional offset current sources toobtain a second magnitude of the offset signal when the second magnitudeis larger than the first magnitude.
 12. The programmable logic circuitof claim 11, wherein the transistor pair comprises a first transistorhaving a gate configured as the first signal input and a drain terminalconfigured as the first signal output, and a second transistor have agate configured as the second signal input and a drain terminalconfigured as the second signal output.
 13. The programmable logiccircuit of claim 11, wherein the plurality of offset current sources arein parallel and each generates a different amount of current.
 14. Theprogrammable logic circuit of claim 11, wherein the plurality of offsetcurrent sources are coupled to either the first signal output or thesecond signal output via a switching circuit.
 15. The programmable logiccircuit of claim 11, further comprising a switching circuit coupledbetween the signal-offset circuit and the transistor pair capable ofcoupling the offset signal to either the first signal output or thesecond signal output.
 16. The programmable logic circuit of claim 11,further comprising a switching circuit coupled between the signal-offsetcircuit and the transistor pair capable of decoupling the offset signalfrom both the first signal output and the second signal output.
 17. Theprogrammable logic circuit of claim 11, further comprising a switchingcircuit capable of coupling the offset signal to either the first signaloutput or the second signal output.